Psalm 143:1-2 (ESV)
A Psalm of David.
Hear my prayer, O Lord;
give ear to my pleas for mercy!
In your faithfulness answer me, in your righteousness!
 Enter not into judgment with your servant,
for no one living is righteous before you.
How can sinners dare go to God and ask Him for mercy? The God that David served was not a God who was unapproachable. David could ask for help from God based on the characteristic of God of faithfulness. Faithfulness to what? Faithfulness to His own promise to be for David. Faithful to the covenant He had made with David. Faithful to His mercies and love for His children.
This ultimately points us to Christ. We can go to God through Jesus Christ and ask God for help (forgiveness, strength, hope, …) because Christ has paid the penalty for our sins, has defeated death for us, has promised to help us. For God not to hear and help us when we come through faith in His Son would be for Him to be unfaithful to what Christ has accomplished for us.
God will never be unfaithful to Christ. We can go to God with great boldness because we have a priest who has offered sacrifice for our sins and enabled us to go. We are not able to ask because of our righteousness, but because of His. We can lose our confidence in being heard and helped by God when Christ ceases to be our faithful high priest and refuses to plead the merits of His death and resurrection to our case. And that will be never.
This is, of course, a blessing only for those who trust Christ. Those who have seen that their righteousness is as filthy rags, who have been convicted of their sin, who have seen Jesus as the only hope for the forgiveness of their sins and who rest in Him alone.
What a God. What a Saviour. What a work He has done for us.